1. Field of the Invention
The present invention relates to a method and apparatus for image forming, and more particularly to a method and apparatus for image forming capable of performing fast and stable sheet transfer operations by using an improved sheet transfer apparatus.
2. Discussion of the Background
FIG. 1 is a schematic cross-sectional view illustrating a background sheet transfer apparatus 1 for use in a background image forming apparatus, such as a printer, a copier, a facsimile, etc. The background sheet transfer apparatus 1 has a sheet passage for a recording sheet P travelling from a sheet tray 2 through an image transfer position where a photoconductor 12 and an image transfer roller 13 are provided. Along the sheet passage, a plurality of rollers and a plurality of sensors are provided. A controller 11, connected to the plurality of rollers and sensors, electrically controls the operation of the background sheet transfer apparatus 1. As shown in FIG. 1, the plurality of rollers include a pick-up roller 3, a feed roller 4a, a reverse roller 4b, a pair of first transfer rollers 5, a pair of second transfer rollers 6, and a pair of registration rollers 10. The pair of first transfer rollers 5, the pair of second transfer rollers 6, and the pair of registration rollers 10 will be referred to as the first transfer roller 5, the second transfer roller 6, and the registration roller 10, respectively. The plurality of sensors include a sensor 7, a transfer sensor 8, and a registration sensor 9. As for these sensors, any kind of reflective sensor is preferably used such as, for example, a photosensor.
In FIG. 1, the sheet tray 2 contains therein a stack of recording sheets P such that their leading edges are substantially aligned at position A of the downstream side of the sheet tray 2. At the starting of a sheet transfer operation, the controller 11 sends a sheet feed signal to the background sheet transfer apparatus 1. With the sheet feed signal, the pick-up roller 3 is rotated and lowered so as to move the recording sheets P to position B where the feed roller 4a and the reverse roller 4b are provided. The feed roller 4a moves forward one of the recording sheets P, while the reverse roller 4b moves back the rest of the recording sheets P. In other words, the recording sheet P placed on the top of the stack is separated from the rest of the recording sheets P, and is transferred to position C where the sensor 7 is provided.
When the sensor 7 detects the leading edge of the recording sheet P at position C, the pick-up roller 3 is lifted and no longer driven. As a result, the recording sheet P is carried by the feed roller 4a to position E where the first transfer roller 5 is provided.
Once the leading edge of the recording sheet P reaches position E, the driving of feed roller 4a is stopped. As a result, the recording sheet P is transferred by the first transfer roller 5, through position G of the transfer sensor 8, to position E′ of the second transfer roller 6.
When the transfer sensor 8 detects the leading edge of the recording sheet P at position G, the controller 11 instructs the background image forming apparatus to start an image writing process on the photoconductor 12. In this example, the first transfer roller 5 and the second transfer roller 6 are driven by a transfer roller driving motor (not shown) controlled by the controller 11.
Subsequently, the leading edge of the recording sheet P is transferred to position I, where the registration sensor 9 is provided. At this time, the registration roller 10 is not driven until the leading edge of the recording sheet P reaches position J where the registration roller 10 is provided. As a result, a skew correction can be performed. That is, the recording sheet P slacks before the registration roller 10 to correct a skew of the recording sheet P if one exists.
After the skew correction, the driving of first and second transfer rollers 5 and 6 is stopped so that a registration correction may take place. That is, the movement of the recording sheet P is timed in synchronization with the rotation of the photoconductor 12 so that the position of the image matches the corresponding position of the recording sheet P.
After the skew correction, the registration roller 10, and the first and second transfer rollers 5 and 6 start rotating. Consequently, the recording sheet P is transferred to position K, where the photoconductor 12 and the image transfer roller 13 are provided, and the image transfer operation is performed.
The background sheet transfer apparatus 1, since the skew correction and the registration correction are performed right before the image transfer operation, temporarily stops the recording sheet P before the registration roller 10. Therefore, the recording sheets P, being continuously fed from the sheet tray 2, are transferred in such a manner that a sheet interval between the trailing edge of a preceding sheet P1 and the leading edge of a succeeding sheet P2 is generated, sufficiently preventing the superposition. In the background sheet transfer apparatus 1, a large sheet interval is provided in consideration of the variation in sheet interval, caused by the variation in recording sheet slippage from position to position along the sheet passage or the variation in the sheet initial position.
The slippage of the recording sheet P varies depending on the relationship between the roller transfer power, in this example, one of the above-described rollers transferring the recording sheet P, and the load being applied by the roller to the recording sheet P. If the transfer power is sufficiently large relative to the load, the recording sheet P can be transferred at a stable speed while causing less slippage. On the-other hand, if the load is sufficiently large relative to the transfer power, the recording sheet P is transferred at a slower speed while causing greater slippage. This relationship between the transfer power and the load varies from position to position in the sheet passage. More specifically, the load applied by the roller varies depending on various conditions including the size, type, or surface of the recording sheet P being transferred. The load is varied depending on the friction coefficient of the roller in use, which is reduced due to wear over time, and deposition of paper dust or foreign substances on the rollers.
As a result, slippage occurs as the recording sheet P passes each roller in the sheet passage, causing the interval sheet variations as explained.
In the background sheet transfer apparatus 1, recording sheets usually experience a large amount of slippage at position B due to the large load generated at the nip between the feed roller 4a and the reverse roller 4b. On the other hand, at position G, where the transfer sensor 8 is provided, the sheets tend to display a smaller amount of slippage. Thus, the background sheet transfer apparatus 1, which typically creates the sheet interval before position B, is likely to have a large sheet interval variation.
In addition, the initial position of the recording sheet P varies from sheet to sheet. Specifically, there is a sheet P transfer delay at the initial position ranging from position A to position B. In consideration of this sheet interval variation generated before position B, the background sheet transfer apparatus 1 typically requires a large sheet interval for a stable sheet transfer operation.
Recently, in order to meet the increased demand for enhanced image forming productivity, image forming apparatuses having a shorter sheet interval are needed. Such image forming apparatuses can achieve higher image forming speeds without increasing roller rotational speeds or requiring high-performance (i.e., high-cost) motors while suppressing motor noise and improving roller durability.
Therefore, in order to develop improved image forming apparatuses with short sheet intervals, sheet interval variations have to be suppressed.